In a Time-of-Flight Mass Spectrometer (TOF-MS) it is advantageous if the ions of a specific mass to charge ratio are accelerated by means of suitable electric fields in such a way that their initial distribution in space is compressed to a thin sheet at the location of the ion detector. The larger initial package contains more ions, while the thin sheets of ions with different mass to charge ratios become well separated, hence sensitivity and resolution are enhanced.
It is taught by Wiley and U.S. Pat. No. 2,685,035 that such a compression can be facilitated by a linear TOF-MS comprising an ion accelerator with one or two stages of homogeneous electric fields, and a drift space which is terminated by the ion detector. Ions that start from a position further back in the accelerator of such a TOF-MS gain more kinetic energy and catch up with those ions that started from a point further forward in the accelerator when they reach the end of the drift space. The compression of the initial spatial distribution in the direction of the axial or longitudinal coordinate is called space focusing or longitudinal focusing.
The focusing achieved by the linear instrument of U.S. Pat. No. 2,685,035 is of first order in the initial longitudinal coordinate, which means, that the flight time is only a quadratic function of the starting position with a minimum or maximum for the middle or reference position. It was found, however, that the mass resolution that can be realized with a linear TOF-MS is limited by the fact, that the ions are not initially at rest but have an initial positive or negative velocity components in the acceleration direction, which results in a dispersion of the ion packages.
Ion reflectors are devices, that can turn around the direction of motion of ions by means of electric fields. Ions penetrate into these fields according to their velocity or energy component in the direction of the reflector field. Ions with higher kinetic energy penetrate deeper and need more time to pass through the reflector. It is therefore possible to achieve energy focusing, which means that the flight times of ions of one mass to charge ratio become largely independent of their initial axial energy.
Traditionally, a high resolution Reflector-TOF-MS is set up in the following way: At first, a primary longitudinal focus is formed close to the beginning of a field free drift space by means of an accelerator with one or two stages. The ions form a thin sheet at the primary longitudinal focus, but have a substantial distribution of axial energies reflecting mainly their different starting position. Then, this primary longitudinal focus is transferred to a secondary longitudinal focus at the location of the ion detector by means of the ion reflector. Ideally, the width of the ion package at the primary focal point is preserved, while the flight path is extended, hence the mass resolution can be higher in a Reflector-TOF-MS.
In a typical system as it was described e.g. by Mamyrin in U.S. Pat. No. 4,072,862, the ion accelerator merely acts as the input stage to the reflector. The geometrical dimensions and the electrical potentials that are required to achieve the primary and secondary longitudinal focus are set up separately for accelerator and reflector, while the individual parts of the Reflector-TOF-MS are connected by the common primary focus.
This route of designing a high resolution Reflector-TOF-MS was modified e.g. by Leisner, who described a TOF-MS comprising a two stage ion accelerator and a two stage ion reflector, which achieved a conceptual longitudinal focusing of first, second and third order. Here, all the electric potentials were determined directly from the equation for the total flight time and the longitudinal focusing conditions.
The two stage Mamyrin ion reflector with homogeneous electric fields provides energy focusing of first and second order, and thus facilitates the highly undistorted transfer of an ion package from the primary to the secondary longitudinal focus. In the design of Leisner a Mamyrin-reflector was used to allow for complete third order space focus at the location of the detector. However, it has the disadvantage, that ions must pass through the meshes of the reflector four times. These meshes reduce the ion transmission and hence the sensitivity of the instrument. They also reduce the mass resolution of the instrument due to scattering of the ions (Bergmann).
On the other hand, the energy focusing boundary condition for a single stage ion reflector requires, that the total field free drift space between the primary and secondary longitudinal focus is four times as long as the mean penetration depth of the ions into the reflector. This results in rather long reflectors, whenever a long flight path is required for high mass resolution. Furthermore, the energy focusing achieved with a single stage mirror is only of first order, thus transfer of the primary focus is less perfect and the overall mass resolution that can be achieved in the conventional way is limited. Ions pass through a single mesh twice on entering and leaving the a single stage reflector. This reduces the ion losses due to scattering, resulting in improved sensitivity when compared to a two stage reflector.